US7292402B2 - Systems and methods for multipass servowriting with a null burst pattern - Google Patents
Systems and methods for multipass servowriting with a null burst pattern Download PDFInfo
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- US7292402B2 US7292402B2 US11/090,608 US9060805A US7292402B2 US 7292402 B2 US7292402 B2 US 7292402B2 US 9060805 A US9060805 A US 9060805A US 7292402 B2 US7292402 B2 US 7292402B2
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/48—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed
- G11B5/58—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed with provision for moving the head for the purpose of maintaining alignment of the head relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
- G11B5/596—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed with provision for moving the head for the purpose of maintaining alignment of the head relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following for track following on disks
- G11B5/59633—Servo formatting
- G11B5/59655—Sector, sample or burst servo format
Definitions
- the present invention relates to servowriting processes, systems, and devices.
- FIG. 1 is a functional diagram showing components of a disk drive that can be used in accordance with embodiments of the present invention.
- FIG. 2 is a diagram showing an example of a data and servo format for a disk in the drive of FIG. 1 .
- FIG. 3 ( a )-( b ) show exemplary 4-burst servo pattern and null burst servo pattern, respectively wherein servo information can be written to the tracks shown in FIG. 2 .
- FIG. 4 ( a )-( c ) show possible sampling locations for DFT processing of burst signals.
- FIGS. 5 ( a )-( d ) are diagrams showing a progression of servowriting passes and steps of servowriting an exemplary null burst pattern.
- FIGS. 6 ( a )-( d ) are diagrams showing a progression of servowriting passes and steps of servowriting an exemplary null burst pattern in accordance with one embodiment of the present invention.
- FIG. 7 shows the flow-chart which can be used to implement another embodiment of the present invention.
- FIGS. 8 ( a )-( c ) are diagrams showing a progression of servowriting passes and steps of servowriting an exemplary null burst pattern in accordance with another embodiment of the present invention.
- FIG. 9 shows the flow-chart which can be used to implement another embodiment of the present invention.
- a typical disk drive 100 includes at least one magnetic disk 102 capable of storing information on at least one of the surfaces of the disk.
- a closed-loop servo system can be used to move an actuator arm 106 and data head 104 over the surface of the disk, such that information can be written to, and read from, the surface of the disk.
- the closed-loop servo system can contain, for example, a voice coil motor driver 108 to drive current through a voice coil motor (not shown) in order to drive the actuator arm, a spindle motor driver 112 to drive current through a spindle motor (not shown) in order to rotate the disk(s), a microprocessor 120 to control the motors, and a disk controller 118 to transfer information between the microprocessor, buffer memory 110 , read channel 114 , and a host 122 .
- a host can be any device, apparatus, or system capable of utilizing the data storage device, such as a personal computer or Web server or consumer electronics device.
- the drive can contain at least one processor, or microprocessor 120 , that can process information for the disk controller 118 , read/write channel 114 , VCM driver 108 , or spindle driver 112 .
- the microprocessor can also include a servo controller, which can exist as an algorithm resident in the microprocessor 120 .
- the disk controller 118 which can store information in buffer memory 110 resident in the drive, can also provide user data to a read/write channel 114 , which can send data signals to a current amplifier or preamp 116 to be written to the disk(s) 102 , and can send servo and/or user data signals back to the disk controller 118 .
- a controller for the data storage device can include the disk controller 128 and/or processor 120 .
- the controller can be on one or multiple chips. In one embodiment, a controller chip contains SRAM while DRAM and FLASH are external to the chip. Other memory arrangements can also be used.
- the information stored on disks can be written in concentric tracks, extending from near the inner diameter (ID) of the disk to near the outer diameter (OD) of the disk 200 , as shown in the example disk of FIG. 2 .
- servo information can be written in servo wedges 202 , and can be recorded on tracks 204 that can also contain data.
- the servo wedges may not extend linearly from the inner diameter of the disk to the outer diameter, but may be curved slightly in order to adjust for the trajectory of the head as it sweeps across the disk.
- FIG. 3 ( a ) One example of a servo pattern is shown in FIG. 3 ( a ). That figure shows a radial extent of several servo tracks, and a circumferential extent of a single wedge.
- the shaded areas represent magnetic media that is magnetized in one direction, and the unshaded areas represent magnetic media that is magnetized in another direction (typically opposite to that of the shaded regions).
- the directions would be in the positive and negative circumferential directions (to the right and left in the figure).
- a perpendicular-recording system the two directions would be perpendicular to the media (in and out of the page in the figure). Both recording systems (longitudinal and perpendicular) can benefit from this invention.
- the read-write head typically stands relatively still while reading and writing data, while the disk moves underneath it, it is often useful to visualize the head passing by regions of the disk as it reads or writes data from or to the disk.
- the head would first encounter the left-most patterns and move to the right across the servo-wedge (while of course, in reality, it is the head that stands still while the servo-pattern on the disk moves to the left).
- the R/W head first encounters the preamble 301 , which is intended to allow the channel to acquire the correct sampling phase and amplification of the servo signal.
- a SAM 302 or “Servo Address Mark”.
- the SAM serves as a timing-mark, relative to which all subsequent information in the servo wedge can be located. Successful identification of the SAM also serves as an indication to the R/W channel that servo demodulation is proceeding properly.
- a servo wedge typically contains digital information about the radial and/or circumferential location of the wedge.
- a short track-number 303 is shown. The track-number typically specifies the general radial location of the wedge. Track-numbers are typically numbered sequentially, increasing either from ID (Inner-Diameter) to OD (Outer-Diameter), or vice-versa.
- the track-number shown in FIG. 3 contains only 4 bits, and can thus uniquely identify only 16 different tracks. Since a modern disk-drive may contain anywhere from 25000 to well over 100000 tracks, many more than 4 bits may be necessary to identify the track-number.
- FIG. 3 contains very few track-number bits primarily to allow easy illustration of a servo wedge.
- the track-number in a servo wedge is typically Gray-coded. Gray-coding of the track-number, as is known to those skilled in the art, ensures that only one bit of the gray-coded value changes from one track to the next. Thus, if the R/W head is between two tracks (as it often is), only one bit of the demodulated track-number might be indeterminate or ambiguous.
- a robust servo-demodulation scheme can deal with one ambiguous track-number bit.
- the track-number identified by the track-number field is a servo-track-number (used, among other fields, by the servo to determine the radial location of the R/W head), which may be different from a data-track-number (which is used to identify a particular track of user-data).
- a servo-track-number used, among other fields, by the servo to determine the radial location of the R/W head
- a data-track-number which is used to identify a particular track of user-data.
- there is a one-to-one correspondence between servo-tracks and data-tracks that is, a given data-track may be associated with a servo-track of the same number, or a servo-track whose track-number is simply offset by a constant value from that of the data-track number).
- a data-track may occupy a larger radial extent than that of a servo-track.
- a popular embodiment involves a ratio of 3/2 between data tracks and servo tracks.
- each data-track occupies the radial extent of 1.5 servo tracks.
- the servo patterns of FIG. 3 do not show any wedge-numbers.
- a servo wedge will have at least some indication of the circumferential location of the wedge, either in the form of a wedge-number included in each wedge, or an “index-mark”, which identifies a single wedge out of all of the wedges of a track as an “index-wedge”, or wedge #0.
- Wedges are typically numbered form 0 to N ⁇ 1 (where N is the total number of servo-wedges on a track) going around the track in the order of encounter of wedges by the R/W head. This practice is well known to one of ordinary skill in the art.
- the wedge-number and/or index-mark are left out of the figures in this document to simplify the figures.
- the servo information often includes bursts of transitions called “servo bursts”.
- a boundary or burst boundary as used herein does not mean or imply that servo bursts forming a boundary necessarily have a substantially common edge as the bursts can be spaced such that there is a gap radially or circumferentially between the bursts.
- the servo information can be positioned regularly about each track, such that when a data head reads the servo information, a relative position of the head can be determined that can be used by a servo processor to adjust the position of the head relative to the track.
- this relative position can be determined in one example as a function of the target location, a track number read from the servo wedge, and the amplitudes and/or phases of the bursts, or a subset of those bursts.
- the number of bursts present in a servo wedge can vary from design to design, ranging from as few as one to as many as 6 or even more bursts in a single wedge.
- the position of a head or element, such as a read/write head or element, relative to a target or desired location such as the center of a track or other desired location, will be referred to herein as position-error.
- Position-error distance may be used to refer to the distance between a target or desired location and an actual or predicted location of a head or element.
- the signal generated as a head or element moves across servo bursts or boundaries between servo bursts is often referred to as a position-error signal (PES).
- PES position-error signal
- the PES can be used to represent a position of the head or element relative to a target location such as a track centerline defined by a mathematical relationship between the amplitudes and/or phases of all of or any subset of the servo bursts.
- FIG. 3 ( a ) shows an exemplary 4-burst servo pattern, which includes A-burst 304 , B-burst 305 , C-burst 306 and D-burst 308 .
- a centerline 300 for a given data track can be “defined” by a series of burst edges or burst boundaries, such as a burst boundary defined by the lower edge of A-burst 304 and the upper edge of B-burst 305 .
- servo demodulation circuitry in communication with the head can produce equal amplitude measurements for the two bursts, as the portion of the signal coming from the A-burst above the centerline is approximately equal in amplitude to the portion coming from the B-burst below the centerline.
- the resulting computed PES can be zero if the track defined by the A-burst/B-burst (A/B) boundary is the center of a data track, or a track centerline.
- the servo controller could direct the voice coil motor to move the head toward the inner diameter of the disk and closer to its desired position relative to the centerline. This can be done for each set of burst edges defining the shape of that track about the disk.
- the four bursts in the servo pattern can be written in multiple servowriting steps one burst at a time.
- each subsequent step writes one servo burst (e.g., 305 ) in a wedge and trims another (e.g., 304 ).
- the servowriting head is stepped by one-half servo track radially, either toward the inner diameter (ID) or outer diameter (OD) of the disk, depending on the radial direction used to write the servo information.
- a seek typically takes anywhere from one quarter to one half of the time it takes for the disk to make one revolution.
- the process of writing the servo pattern for each step typically takes one or two full revolutions (passes) to write all of the wedges in that pass. It is possible that completing the burst writing and trimming for a single servowriting step can take more than two revolutions, but a maximum of two revolutions (one to write the new burst, and another to trim a previously-written burst) will be considered for the discussion below.
- servowriting can take about 1.25-2.5 revolutions per servowriting step. Since there are two servowriting steps per servo-track in this example, and 1.5 servo tracks per data-track, such a process requires 3 servowriting steps per data-track, or 3.75-7.5 revolutions per data-track. For purposes of subsequent discussion only, it will be assumed that the process takes 4 revolutions per data-track (a relatively low bound).
- a disk drive can have tens of thousands of data tracks. With 100,000 data-tracks and a spin-speed of 5400 RPM (90 Hz), for example, the process would take 4,444 seconds, or about 75 minutes. If the process is carried out on an expensive servowriter, this can add substantially to the cost of the drive. Thus, drive manufacturers are motivated to use self-servowriting techniques to reduce or eliminate the time that a drive must spend on a servowriter.
- a measurement can be made to characterize servo loop characteristics, which can be combined with the observed PES in order to determine the written-in runout of the reference track. Because the servo typically suffers both synchronous and non-synchronous runout (sometimes referred to in the industry as “repeatable” runout (RRO) and “non-repeatable” runout (NRRO), respectively), any measurement intended to determine the synchronous runout can be affected by the non-synchronous runout. If many revolutions of PES data are observed and combined (one possible approach to combine is to synchronously average the PES data, other possible approaches are outlined in U.S. Pat. Nos.
- FIG. 3 ( b ) shows an exemplary null burst servo pattern, which includes A-burst 308 and B-burst 309 are radially displaced from each other by one servo track at approximately the same space.
- C-burst 310 and D-burst 311 can be similarly written in quadrature with the A/B burst, i.e., it would have its center point at the same radial location where the A/B burst has a max or min value.
- the null burst pattern is shorter in length and takes less space on the servo track.
- the two bursts are typically demodulated via DFT (Discrete-Fourier Transform) processing of digitized, filtered samples of the signal from the R/W head as it passes over the bursts.
- DFT Discrete-Fourier Transform
- a drive system can use an algorithm that computes the real and imaginary parts of the DFT of the burst signal.
- Existing channels are capable of sampling the signal and doing a discreet Fourier transform.
- One such discreet Fourier transform that can be used is given as follows:
- ⁇ n the sequence in time
- F k the Fourier component in frequency space.
- FIGS. 4 ( a ), 4 ( b ), and 4 ( c ) show example burst-signals with three different phases, relative to sample-times (each of which is marked with an “*” in those figures).
- the signal in FIG. 4B is slightly phase-advanced, relative to the one in FIG. 4 ( a ).
- the signal in FIG. 4B is slightly phase-advanced, relative to the one in FIG. 4 ( a ).
- a discreet Fourier transform of the signal then can be broken down into real and imaginary parts, which can each be squared and added together. The square root of this sum yields the magnitude of the signal. If the nominal sampling times of the bursts is as shown in 4 ( a ), and the alternative coefficients described above (+1, ⁇ 1, ⁇ 1, +1 for real part and +1, +1, ⁇ 1, ⁇ 1 for imaginary part) are used to process the signal, then the imaginary part of the DFT may be used to indicate the signed amplitude of the burst (which is useful in computing the PES for a null-burst pattern, as is described in U.S. Pat. No.
- a linear combination of the real and imaginary parts of the DFT can be used to indicate the signed amplitude of the bursts.
- the nominal sampling times shown in 4 ( a ) are to be used, along with the alternative coefficients described above, so that the imaginary portion of the DFT of the sampled burst signal is a good indication of the signed amplitude of the bursts.
- FIGS. 5( a )- 5 ( d ) depict the progression of several servowriting passes and steps of servowriting an exemplary null burst pattern, which is known in the art.
- the burst pattern is named “null” because there is no signal when the A-burst and B-burst (as described later) are straddled together and are 180 degrees out of phase with each other.
- the signed amplitude instead of either the absolute amplitude or the phase alone
- the real or imaginary parts of the DFTs of the burst signals or a combination of both the real and imaginary parts will be used to determine the demodulated position during demodulation of the pattern.
- FIG. 5 ( a ) the result of a single servowriting step is shown. From that step, the servowriting head 500 has written an exemplary A-burst 501 .
- FIG. 5 ( b ) shows the result of the first pass in the second servowriting step by the servowriting head, which trimmed the A-burst and wrote a B-burst 502 .
- 503 shows the portion of the A-burst that has been trimmed during this step and 504 is the boundary defined by lower edge of the A-burst and the upper edge of the B-burst.
- the B-burst is one servo track displaced radially from A-burst at the approximately the same place (and time slot), but is 180 degrees (or ⁇ radians) out of phase from the A-burst, where 360 degrees (or 2 ⁇ radians) represents one (approximately) sinewave cycle of the signal.
- the advantage of such null pattern is that two bursts can be written in the space of one, so that more information can be stored with less noise on the disk.
- trimming and writing operation are performed in a single pass at exactly the same time instead of separate passes, the “separate-trim-and-write” technique (as described in U.S. Pat. No.
- One way to resolve the high RRO issue is to write B-burst and trim A-burst twice in the same (second) servowriting step so that the RRO on two passes can be randomized. If the position of the write head in the second pass (of writing B-burst) is below where it was on the first pass, there is little impact on the B-burst already written as shown in FIG. 5 ( c ). If, however, the position of the write head in the second pass is higher than where it was on the first pass, as shown in FIG. 5 ( d ), the A-burst will be re-trimmed and the boundary 505 defined by A/B burst will be re-drawn as if the first pass of trimming and writing had never happened.
- A-burst 601 in FIG. 6 ( a ) is written in the first servowriting step in the exactly the same way as A-burst 501 in FIG. 5 ( a ).
- a first portion for example, half or slightly more than half
- the whole A-burst 602 and B-burst 603 are trimmed and written respectively in the same time slot, as shown in FIG. 6 ( b ).
- the second portion of the A-burst 604 and B-burst 605 are then trimmed and written respectively, as shown in FIG. 6 ( c ). If the position of the write head during the second pass is below where it was during the first pass, as shown in FIG. 6( c ), the second portion of transition pairs (composed of A and B-bursts) will be affected except for the transition pairs 606 in the middle (which were trimmed/written during the first pass) that will remain intact. If the position of the write head during the second pass is above where it was during the first pass, as shown in FIG. 6( d ), then all the second portion of transition pairs are affected including the transition pairs 607 in the middle.
- the RRO distribution of the bursts will be similar to that by the “separate trim and write” approach without the “long tail” and the RRO of bursts written by the two passes can be randomized and better RRO can be obtained compared to the previous approaches. This idea can also be extended to more than two sub-bursts, as would be apparent to one of skill in the art.
- FIG. 7 shows a flow-chart that can be used for servowriting the null burst pattern in accordance with the present invention. Although only the servowriting of A and B-bursts is described in this and the following Figures, C and D-bursts can be handled in similar fashion, which is obvious to one of ordinary skill in the art.
- an A-burst on a track can be written via a servowriting head during the first servowriting step.
- a first portion of the A-burst is trimmed and a first portion of a B-burst is written in the same time slot one servo track radially displaced and 180 degrees out of phase from the A-burst at step 704 .
- a second portion of the A-burst is trimmed and a second portion of a B-burst is written at step 706 , while the position of the write head may be slightly above or below where it was during the first pass of the second step. Plus, the first and second portion of A-burst and B-burst may overlap and some transition pairs may be written or trimmed twice.
- the first and second portions of the transition pairs of the bursts are demodulated as a single burst to calculate a PES at step 708 . If it was possible to write/trim exactly half of the burst in each separate revolution, then this would be the best strategy. However, it would be better to overlap the write/trim operations than to leave some portion of the B-burst un-written and a corresponding portion of the A-burst un-trimmed (which could cause a large level of written-in runout). Thus, since it is impossible to write/trim exactly half of the burst in each operation, given the choice, it is better to overlap the operations than to “underlap” them.
- FIG. 8( a )-( c ) Another embodiment of the systems and methods of the present invention is shown in FIG. 8( a )-( c ).
- A-burst 801 in FIG. 8 ( a ) is written in the first servowriting step in the exactly the same way as A-burst 501 in FIG. 5 ( a ) and A-burst 601 in FIG. 6 ( a ).
- the first pass of the second servowriting step is a trimming-only pass on A-burst written in the first step.
- a higher erase current than what will be used for writing the B-burst in the next pass is used for the trimming, so that A-burst 802 will be erased higher up than the B-burst that is to be written next will extend, as shown in FIG.
- Such an approach intentionally creates a gap between the lower edge of the A-burst 802 and the upper edge of the B-burst 804 , and as long as the difference between the erase and write currents on the two passes is sufficient to displace the corresponding trim and write edges by more than the potential “jitter” in the head position during writing, there will be a separately determined lower edge of the A-burst 802 and upper edge of the B-burst 804 . Since the radial locations of the two edges should be relatively un-correlated, this approach will have the same advantage of typical “separate trim/write” approaches and it is capable of reducing the RRO of the tracks on the disk.
- FIG. 9 shows a flow-chart that can be used for servowriting the null burst pattern in accordance with the present invention.
- An A-burst on a track can be written via a servowriting head during the first servowriting step at step 902 .
- the A-burst is trimmed higher up than the B-burst that is to be written next will extend via a high erase current at step 904 .
- a B-burst is written with a lower write current one servo track radially displaced and 180 degrees out of phase below the A-burst at step 906 .
- the two bursts are demodulated to calculate a PES and determine the position of the head at step 908 .
- embodiments described herein refer generally to systems having a read/write head that can be used to write bursts on rotating medium (magnetic media), similar advantages can be obtained with other such data storage systems or devices.
- a laser writing information to an optical media can take advantage of additional passes when writing position information.
- Any media, or at least any rotating media in a single and/or multi-headed disk drive, upon which information is written, placed, or stored, may be able to take advantage of embodiments of the present invention.
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Description
In this equation, ƒn is the sequence in time and Fk is the Fourier component in frequency space. This “complex” math can be simplified in at least a few situations. For example, a signal can be examined at one quarter of the sample rate. In that case, the frequency-index, k, of interest would be k=M where M is the number of (approximately) sinewave cycles processed for each burst. For example, if the servo burst was 8 complete cycles in length (as is the case in
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Cited By (2)
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US20070230030A1 (en) * | 2005-01-12 | 2007-10-04 | Kabushiki Kaisha Toshiba | Magnetic recording medium and magnetic recording/reproducing apparatus |
US20140268397A1 (en) * | 2013-03-15 | 2014-09-18 | Lsi Corporation | Hardware Support of Servo Format with Two Preamble Fields |
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JP4358067B2 (en) * | 2004-08-06 | 2009-11-04 | 株式会社東芝 | Magnetic recording medium and magnetic recording apparatus |
JP4291784B2 (en) * | 2005-01-12 | 2009-07-08 | ヒタチグローバルストレージテクノロジーズネザーランドビーブイ | Servo information recording method, magnetic recording medium, and magnetic disk apparatus |
JP4649262B2 (en) * | 2005-04-19 | 2011-03-09 | 株式会社東芝 | Method for manufacturing magnetic recording medium |
JP4585476B2 (en) | 2006-03-16 | 2010-11-24 | 株式会社東芝 | Patterned medium and magnetic recording apparatus |
JP4675812B2 (en) * | 2006-03-30 | 2011-04-27 | 株式会社東芝 | Magnetic recording medium, magnetic recording apparatus, and method of manufacturing magnetic recording medium |
US7265937B1 (en) * | 2006-06-09 | 2007-09-04 | Seagate Technology Llc | Positioning of a head array over a data storage medium |
JP2008282512A (en) * | 2007-05-14 | 2008-11-20 | Toshiba Corp | Magnetic recording medium and magnetic recording/reproducing device |
JP4382843B2 (en) * | 2007-09-26 | 2009-12-16 | 株式会社東芝 | Magnetic recording medium and method for manufacturing the same |
JP4858919B2 (en) * | 2008-05-09 | 2012-01-18 | 東芝ストレージデバイス株式会社 | Storage device, storage device control circuit, and servo write method identification method |
US9053728B1 (en) | 2014-11-21 | 2015-06-09 | HGST Netherlands B.V. | Servo systems with PES enhanced integrated servo bursts |
US11830524B1 (en) | 2022-06-13 | 2023-11-28 | Western Digital Technologies, Inc | Data storage device with split burst servo pattern |
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US6738205B1 (en) * | 2001-07-08 | 2004-05-18 | Maxtor Corporation | Self-writing of servo patterns in disk drives |
US20040123025A1 (en) * | 2002-12-18 | 2004-06-24 | Chainer Timothy J. | Radial self-propagation pattern generation for disk file servowriting |
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US6738205B1 (en) * | 2001-07-08 | 2004-05-18 | Maxtor Corporation | Self-writing of servo patterns in disk drives |
US20040123025A1 (en) * | 2002-12-18 | 2004-06-24 | Chainer Timothy J. | Radial self-propagation pattern generation for disk file servowriting |
Cited By (3)
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US20070230030A1 (en) * | 2005-01-12 | 2007-10-04 | Kabushiki Kaisha Toshiba | Magnetic recording medium and magnetic recording/reproducing apparatus |
US20140268397A1 (en) * | 2013-03-15 | 2014-09-18 | Lsi Corporation | Hardware Support of Servo Format with Two Preamble Fields |
US8970981B2 (en) * | 2013-03-15 | 2015-03-03 | Lsi Corporation | Hardware support of servo format with two preamble fields |
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